Still, background checks are one of the latest battlegrounds in the history of gun laws in the United States, where residents own more firearms per capita than any other country in the world.

Advocates of gun control have called for bans on certain weapons or buybacks to reduce the number in circulation, while proponents of second amendment rights decry government attempts to restrict access to arms.

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So what do the current laws actually allow? And how did it get to be that way? Long story.

"There's not a commonality among states about gun laws. They're all over the place," said Melissa Hamilton, a former Florida police officer and current professor of criminal law at the University of Houston Law Center.

Not surprisingly, Texas trends towards the hands-off end of firearm philosophy. It puts almost no regulation on gun owners. But that doesn't mean there are no regulations.

Despite the spectrum of state laws, they do share one commonality: federal law.

Federal gun laws

The U.S. Constitution, ratified 1788, made mention of firearms in the Second Amendment, which reads, "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms shall not be infringed."

At that time, the vast majority of firearms were long, single-shot rifles.

In 1929, a gang-related massacre of seven men with Thompson sub-machine guns in a Chicago warehouse brought to national attention the prominence of high-powered weapons in organized crime.

Congress reacted with the National Firearms Act in 1934. It required registry and taxation of firearms used primarily for criminal purposes, including sawed-off shotguns, machine guns and silencers.

The court denied that the weapon in question, a sawed-off shotgun, "has today any reasonable relation to the preservation or efficiency of a well regulated militia, and therefore cannot say that the Second Amendment guarantees to the citizen the right to keep and bear such a weapons."

Firearms used int he 1999 Columbine massacre included a sawed off shotgun, which is illegal under a 1934 law.

Mentally ill people, either involuntarily committed to a mental health institution or ruled by a court unfit to live alone

People dishonorably discharged from the U.S. military

People who have renounced their U.S. citizenship

Undocumented immigrants

People subject to restraining orders against "an intimate partner or child of the intimate partner"

People who have been convicted of domestic violence

Those bans were difficult to uphold, because they were based on an honor system—a gun buyer should disclose any disqualifying credentials.

The law was updated in 1986, when Congress under President Ronald Reagan passed the Firearm Owners' Protection Act. It was meant to protect legal gun owners from burdens of federal regulation. It strengthened and weakened parts of the 1968 law.

It allowed licensed dealers to operate away from their business, such as at gun shows.

It made narrow and vague the definition of a firearms dealer as someone who devotes "time, attention, and labor in dealing in firearms as a regular course of trade or business with the principal objective of livelihood and profit through the repetitive purchase and resale of firearms."

Previously, anyone who sold firearms was considered a dealer. The narrower definition of that term meant that small-scale sellers could operate without a federal license, though the law also banned them from selling to prohibited customers.

Under the law, sellers could only be held liable for selling to prohibited customers if prosecutors proved they knew the customer was not eligible to buy.

That was hard to prove, since buyers could lie about their criminal background.

The background check requirement did not apply to unlicensed sellers or to sellers at a gun show.

In 1994, Congress passed the Violent Crime Control and Law Enforcement Act, which included a 10-year ban on manufacture of "assault weapons"—certain semi-automatic or automatic long guns. But the law only applied to guns produced after 1994, and it expired in 2004 without renewal.

"Assault rifle" is a broad term used for a magazine-fed semi-automatic or automatic rifle. Their manufacture was banned in the U.S. from 1994 to 2004.

The Supreme Court issued a second landmark decision on gun rights in 2008. In hearing a case over the legality of a handgun, it reversed the court's 1939 interpretation of the constitution, ruling that "the Second Amendment protects and individual right to possess a firearm unconnected with service in a militia."

So those are the basics of federal gun law. How about Texas?

Texas law

Randal County delegate Andrew Michael Holley open carries a handgun during the Texas Republican Convention on May 13, 2016, in Dallas. (AP Photo)

Photo: LM Otero, STF

The Lone Star State puts no additional restriction on gun ownership. It deals primarily with licensure—issuing permits for residents to carry handguns in public. Long guns have always been permitted in public.

Previously, those handguns carried in public had to be concealed. In 2015, the state legislature voted to allowed the open carrying of handguns. It also voted to allow handguns on college campuses, where they had previously been banned.

Anyone can sell a firearm online or at a gun show without a license, and they need not conduct a background check. The same is true in 31 other U.S. states.

There are no regulations on ammo sales in Texas, or on volume of firearm sales.

"Dealers have discretion," said Andrea Brauer, executive director of the group Texas Gun Sense. "If they see someone coming in everyday buying 20 guns, they can refuse to sell."

Hamilton at the University of Houston said cities play virtually no role in firearm regulation, especially since the 2008 Supreme Court case struck down a virtual handgun ban imposed by Washington D.C.

But most cities will keep basic gun laws on the books--such as no firing a weapon in the city limits.